When electrical cables are inserted into an electrical box, such as a junction box, metal or plastic traps are used to secure the cables to the box. This prevents longitudinal forces from stressing wire connections in the box and separating the connections.
When a metal trap is used, a pre-punched knockout in the side of the box is removed and the metal trap is inserted in the resulting hole. The trap then is secured in place using a threaded nut in the interior of the box. The electrical cable is then threaded through the trap. Next, the wires of the cable are stripped and any excess cable is backed out of the electrical box. Two screws on the metal trap then are tightened to secure the electrical cable in place. Finally, the wires of the cable are connected and a cover is placed on the electrical box.
When a plastic trap is used, a pre-punched knockout is removed, and the plastic trap is placed around the cable. The plastic trap then is inserted into the hole in the electrical box corresponding to the knockout. The wires then are stripped and connected. Finally, the cover is placed on the electrical box.
A representative electrical cable in both of these examples is a Romex.RTM. cable, also known as non-metallic sheathed cable (type NM-B). Such a cable includes multiple conductors and an outer plastic protective sheath.
In both examples, an external trap is positioned in a knockout hole to ensure that the electrical cable remains in place. Normally, the external sheath of the cable and the insulation around individual conductors are removed after the cable is passed through the trap.